OC Q: Printing a 26" line actually ends up measuring 25 and 15/16". Why?


Raster images are measured/defined in pixels. Not inches. (There is no size in inches stored anywhere.)

  1. You can only have a whole number of pixels. So you can have an image that is 100x100 px, but not 100.3x100.5 px or whatever.

  2. An image file may have a PPI setting attached. This is a number that states how many Pixels Per Inch there should be - a conversion factor for calculating print dimensions. (Many people refer to it as "DPI").

These two taken together means your "physical" dimensions will end up with rounding errors. The lower the PPI, the larger the rounding errors.

Some simplified/exaggerated example values as demonstration:

  • 5x5 pixel image.
  • Set to 5 pixels/inch.
  • This gives us an image that is 1x1 inch.

If we then try to set the image size to be 2x2 inches (without resampling), it will need a PPI of 2.5. When you confirm, this gets rounded to 3 (as it has to be an integer). 5 px @ 3 PPI is 1.6667 inches. And if you reopen Image Size you will indeed see this, and not the 2 inches that you entered!

(If the image was 10x10 px, then we could get exactly 2x2 inches by having a PPI of 5.)

Now, in a print dialog, or in layout software, you should be able to work with inches as units (ignoring the PPI value of the image file, and choosing whatever print size you want).

Ps: Note that printers can also be a tiny bit inaccurate. Paper can even expand/contract disproportionally throughout a printing process! (This will naturally vary a great deal depending on the printing method and paper/substrate.)

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